In the field of microfinishing using ultraviolet light, such as with semiconductor exposure apparatuses, strict exposure control is necessary in order to maintain the resolution of the circuit pattern at a constant level or better. However, excimer lasers used as the light source in semiconductor exposure apparatuses have variations in the pulse energy of each pulse since these lasers are so-called pulse excitation gas discharge lasers. It is necessary to reduce these variations in order to improve the precision of exposure control.
Therefore, a method for decreasing the variation in the cumulative value of the irradiated energy, by decreasing the light energy output with one pulse oscillation and irradiating the same location to be machined with a plurality of successive pulses, is being considered.
In consideration of production, however, a large number of pulses is not preferable. Also, in the field of semiconductor exposure, the sensitivity of photosensitive materials applied to wafers has improved in recent years and exposure with a small number of pulses is becoming possible. For this reason, a method to increase the number of pulses and reduce variations in total energy of the irradiated light is unavoidable.
However, semiconductor exposure apparatuses repeatedly alternate between exposure and stepped movement. Therefore, the operating state of the excimer laser, the light source, as shown in FIG. 23, necessarily becomes the burst mode of switching between the action of continuous pulse oscillation to oscillate the laser beam continuously at a fixed frequency, and the action of stopping the pulse oscillation for a prescribed time. In effect, the burst mode switches alternately between the continuous pulse oscillation period and the oscillation stoppage period. In effect, in FIG. 23, one IC chip formed on a semiconductor wafer undergoes machining with a pulse group constituting a single continuous pulse oscillation period. Moreover, FIG. 23 shows the energy intensity of each pulse when the excitation intensity (charging voltage) is set at a constant value.
As discussed above, since the excimer laser is a pulse excitation gas discharge laser, it is difficult for it to continuously oscillate at a pulse energy of a constant size. The cause of this is as follows: the discharge disturbs the density of the laser gas within the discharge space, making the next discharge irregular and unstable; because of this irregular discharge, etc., a localized temperature increase occurs on the surface of the discharge electrode, deteriorating the next discharge; and discharge becomes irregular and unstable.
In particular, that trend is marked in the initial phase of the continuous pulse oscillation period; the so-called spiking phenomenon appears as shown in FIGS. 23 and 24. In the spiking phenomenon, a comparatively high pulse energy is attained initially in the spike zone, including the first few pulses after the oscillation stoppage period t, and afterwards pulse energy gradually decreases. When this spike zone is finished, the pulse energy passes through a plateau zone where a stable value at a comparatively high level is maintained, and then enters a stable zone (stationary zone).
This type of excimer laser apparatus with a burst mode operation has such problems as variations in the energy of each pulse, discussed above, decreasing the precision of exposure control, and the spiking phenomenon markedly enlarging the variations further and greatly reducing the precision of exposure control.
Moreover, in recent years, the sensitivity of photosensitive materials applied to wafers has increased, as discussed above, making possible exposure with a small number of continuous pulses; the trend is for a reduced number of pulses. However, variations in pulse energy have accordingly increased as the number of pulses has decreased. It has become difficult to sustain the precision of exposure control through only the multiple pulse exposure control discussed above (control by reducing the amount of energy output in a single pulse oscillation and irradiating the same location to be machined with a plurality of successive pulses).
Therefore, the present applicant is applying for patents for various inventions using the property that pulse energy increases as excitation intensity (charging voltage) increases and relating to so-called spiking prevention control, to prevent the initial energy increase due to the spiking phenomenon, by changing the discharge voltage (charging voltage) for each pulse, through reducing the discharge voltage (charging voltage) of the initial pulse in continuous pulse oscillation in burst mode and gradually increasing the discharge pulse (charging voltage) of subsequent pulses, as shown in FIG. 25 (Japanese Patent Application No. 4-191056, Japanese Patent Application Laid-open No. 7-106678 (Japanese Patent Application No. 5-249483), etc.).
Specifically, FIG. 26 shows the energy intensity of each pulse in the case of excitation intensity (charging voltage) being fixed at a constant value, as shown in FIG. 23 or 24 noted above. A spiking phenomenon is exhibited wherein a comparatively high pulse energy is attained initially at a constant state of excitation intensity and pulse energy gradually decreases thereafter.
FIG. 25 shows an excitation intensity pattern in the case of the spiking phenomenon occurring as shown in FIG. 26. The excitation intensity pattern shows the excitation intensity displacement to correct the increased energy of the initial pulse in the spiking phenomenon and attain a constant pulse energy value. This instance shows the excitation intensity pattern with pulse energy conversion. In other words, because pulse energy is high for the first few pulses in the spiking phenomenon as shown in FIG. 25, the excitation intensity is reduced for the first few pulses in continuous pulse oscillation, and then the excitation intensity is gradually increased. In this way, during pulse oscillation, the source voltage is applied according to this excitation intensity pattern for each pulse oscillation. This prevents the initial rise in pulse energy due to the spiking phenomenon and controls so that the pulse energy of each pulse is the same for all pulses.
This background art has source voltage data to set the energy of each pulse in continuous pulse oscillation to a desired target value (constant value), in view of various parameters such as the oscillation stoppage time t (See FIG. 23) and power lock voltage (source voltage determined according to the deterioration of laser gas), stored in memory in advance for each pulse in continuous pulse oscillation. Meanwhile, this background art detects the pulse energy during continuous pulse oscillation conducted up to the previous time, compares this detected value and the pulse energy target value, and corrects the previously stored source voltage data corresponding to each pulse based on the results of this comparison. This correction is called spike killer control.
However, this background art gives rise to the following problems; the measures to prevent the spiking phenomenon are not necessarily sufficient.
The problems are explained using FIG. 27.
FIG. 27 shows the pulse waveform during burst mode operation in semiconductor exposure. In the figure, No. 1, No. 2, . . . No. j, . . . No. N are pulse groups. Each pulse group is constituted of a prescribed number of continuous pulses as shown in FIGS. 23, 24, and 26. A short laser oscillation stoppage time .DELTA.Tj appears between No. j and No. j+1. A long laser oscillation stoppage time .DELTA.T appears after the No. N pulse group.
This type of arrangement of pulse groups derives from semiconductor exposure being conducted while the process alternates between exposure of the chip on the wafer and movement of the optical system. Specifically, the action of exposing one particular chip on a wafer is conducted with the No. j pulse group and the action of exposing the next chip is conducted with the No. j+1 pulse group. The time necessary for exposure and movement the optical system is .DELTA.Tj. The exposure of a single wafer is entirely finished at the time when the switching between this exposure and movement of the optical system and the oscillation of the series of pulse groups from No. 1 to No. N is completed. Here, .DELTA.T is the time in which the exposed wafer is transported out and the next wafer is transported into the exposure apparatus and aligned in a position where exposure is possible. After this .DELTA.T, the series of pulse groups No. 1', No. 2', . . . No. j', . . . No. N', which is the same as No. 1 to No. N, continues.
In the case of the laser operation discussed above, the excitation intensity pattern of the No. 1 pulse group, which is directly preceded by the same laser oscillation stoppage time (.DELTA.T in this case), is used in the oscillation of the next No. 1' pulse group after .DELTA.T following the completion of No. N, in order to suppress the spiking phenomenon occurring in each pulse group. Also, the excitation intensity pattern of the No. N pulse group, which is directly preceded by the same laser oscillation stoppage time, is used for the No. 2' to No. N' pulse groups (.DELTA.T to .DELTA.TN-1 is the same length of time). In other words, the data for the previous pulse group No. 1 is used for the initial pulse group No. 1' since the influence of the spiking phenomenon is marked, but because the influence of the spiking phenomenon gradually decreases, the No. N pulse group data is used for the pulse groups No. 2' and later for simple control.
The influence of the spiking phenomenon in each pulse group is suppressed to a certain extent. However, the experiments of the present inventors show that the variations in pulse energy are not necessarily resolved due to the cause discussed below and the effect of the suppression is not stable.
The cause is that the spiking phenomenon is influenced by the hysteresis of prior pulse oscillation. In other words, the spiking phenomenon has the property of becoming more marked when the laser oscillation stoppage time in burst mode is greater. Therefore, the following phenomenon occurs: the suppression of the spiking phenomenon easily becomes insufficient in the first half, No. 1', No. 2'. . . , of the series of pulse groups No. 1' to No. N' following No. 1 to No. N in FIG. 27, and the effects of suppressing the spiking phenomenon are sufficiently displayed in the second half of the pulse groups, . . . No. N'-2, No. N'-1, No. N'. In this way, the excitation intensity pattern of a pulse group is different depending on where the pulse group falls within the series of pulse groups. The suppression of the spiking phenomenon does not take effect even with the application of data for a pulse group directly preceded by the same laser oscillation stoppage time.
Consequently, because pulse oscillation is controlled using the preceding excitation intensity pattern, variations in pulse energy are not resolved with a conventional laser apparatus as represented in the Japanese Patent Application No. 4-191056 and suppression of the spiking phenomenon is still desirable.
Also, with the background art, the suppression of variations in pulse energy is insufficient in zones other than the spike zone, since spike killer control is performed in the plateau zone and stable zone as well as in the spike zone shown in FIG. 24. Moreover, the suppression of pulse energy is insufficient even when spike killer control is performed only in the spike zone and plateau zone.
This is thought to be due to the following: the influence of the laser oscillation stoppage (laser is stabilized) remains strong in the initial period of continuous pulse and the output power becomes high compared to other zones even if the same source voltage is impressed; and in the subsequent plateau zone and stable zone, the influence of laser oscillation stoppage decreases, while the influence of pulse oscillation (increased electrode temperature, turbulence of laser gas, etc.) is even stronger.
Also, with the background art, the amount of data stored to effect spike killer control for all pulses in continuous pulse oscillation becomes great. This gives rise to the following problems: a large memory capacity and time to read data from memory are required.
However, as memory capacity is increased, the exposure system for apparatuses for exposing semiconductors will change from a stepper system, for stopping the stage and effecting exposure, to a step and scan system, for effecting exposure while moving the stage. The advantage of this step and scan system is that large areas can be exposed. For example, when using a lens with a field size of 36 mm, the exposed area is a 25 mm square in the stepper system, but the step and scan system makes possible the exposure of a large area of 30.times.40 mm. In the future, chip sizes will increase as the degree of integration increases and high precision exposure with the step and scan system will be desired.
In other words, with the step and scan system, machining is effected while zones irradiated by pulse laser beams on a machined item are staggered at a prescribed pitch each time a pulse laser is radiated, so that a prescribed number NO of pulse lasers, each established in advance, irradiate all points on the machined item. In this step and scan system, however, it is difficult to effect control in such a manner that the exposure of each point on a machined item is the same because the pulse laser beam is scanned continuously. Therefore, an effective control method is desirable.
It is an object of the present invention to provide a laser apparatus, being a laser apparatus operated in the burst mode, wherein the influence of the spiking phenomenon is eliminated as much as possible, so as to further improve the precision of machining with a laser beam.
Also, it is an object of the present invention to provide a laser apparatus which can make uniform the exposure of each point on a machined item in the case of effecting machining with the step and scan system.